Method and system for multi-mode mechanical resonator

ABSTRACT

Provided is a fingerprint sensor including one or more mechanical devices for capturing the fingerprint. The resonators are configured to be mechanically damped based upon an applied load.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to biometrics.

2. Related Art

A variety of well known techniques exist for sensing, measuring, andidentifying biometric characteristics. These techniques focus on uniquecharacteristics associated with structures that form the biometrics. Byway of example, fingerprints, defined by ridges and valleys in a finger,are one such biometric.

As known to those of skill in the art, fingerprints are defined byunique structures on the surface of the finger called ridges andvalleys. These ridges and valleys can be sensed, measured, andidentified based upon a number of different modalities.

For example, some fingerprint measurement modalities rely on densityvalues associated with the ridges and valleys. Others rely on dielectricpermittivity as measured when an electric current if passed through theridges and valleys. With respect to dielectric permittivity for example,the permittivity of a ridge (i.e., fingerprint tissue), is differentfrom permittivity of a valley (i.e., air between the ridges).

Capacitive sensing is one technique that can be used to detect changesin permittivity. With capacitive sensing, capacitance values generatedwhen a sensor plate (electrode) touches a ridge are different than thosegenerated when the sensor is exposed to a valley.

Yet another modality is thermal conductance which is a measure of thetemperature differences between the ridges and valleys. Optics are yetanother modality. Optical techniques rely on an optical index ofrefractive and reflective changes between the ridges and the valleys.

Although the modalities differ, each approach seeks to accuratelydistinguish ridges from valleys in order to image the fingerprint. Somemodalities, or techniques, are inherently more accurate that othersrelative to distinguishing ridges from valleys, as will be discussedmore fully below. A relative assessment of this accuracy can becharacterized in terms of contrast ratio. In a biometric context,contrast ratio is a measure of the contrast between tissue (i.e.,fingerprint ridge) to air (i.e., fingerprint valley).

Viewed from another perspective, contrast ratio is an objective way toquantify potential differences in accuracy between thermal basedmodalities, those that rely on dielectric permittivity, from opticalbased modalities and others. The higher the contrast ratio, the greaterpotential for more accurate biometric sensing. When constructing asensing system that incorporates, for example, one of the modalitiesnoted above, designers must consider not only contrast ratio, butmanufacturability, along with costs.

Thermal and dielectric permittivity based sensing systems, for example,have relatively low contrast ratios, as will be discussed more fullybelow. That is, under ideal conditions and with the utmost care andconsideration during design and/or manufacturing, these systems areinherently limited in the accuracy of their measurement output data.

What is needed, therefore, are highly reliable techniques for sensingbiometrics. What are also needed are techniques for sensing biometrics,such a fingerprint ridges and valleys that have higher contrast ratiosthan traditional sensing systems.

BRIEF SUMMARY OF THE INVENTION

In one embodiment, the present invention includes a fingerprint sensorincluding one or more mechanical devices for capturing the fingerprint.The resonators are configured to be mechanically damped based upon anapplied load.

Additional features and advantages of the invention will be set forth inthe description that follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theadvantages of the invention will be realized and attained by thestructure and particularly pointed out in the written description andclaims hereof, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable one skilled in the pertinent art to make and usethe invention.

FIG. 1A is an illustration of the basic structure of a fingerprint;

FIG. 1B is a more traditional view of the fingerprint structuresillustrated in FIG. 1A;

FIG. 2 is a tabular illustration conveying contrast ratios amongdifferent biometric sensing techniques;

FIG. 3 is a graphical illustration of the contrast ratios of the sensingtechniques shown in FIG. 2;

FIG. 4A is an illustration of an exemplary pillar matrix arranged inaccordance with an embodiment of the present invention;

FIG. 4B is an illustration of an exemplary matrix having pillarsconnected diagonally;

FIG. 4C is an illustration of the matrix of FIG. 4B having passivesurrounding pillars;

FIG. 5 is an illustration of elements of an acoustic impedance sensingsystem constructed in accordance with embodiments of the presentinvention; and

FIG. 6 is an exemplary method of practicing an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the present invention refers tothe accompanying drawings that illustrate exemplary embodimentsconsistent with this invention. Other embodiments are possible, andmodifications may be made to the embodiments within the spirit and scopeof the invention. Therefore, the following detailed description is notmeant to limit the invention. Rather, the scope of the invention isdefined by the appending claims.

It will be apparent to one skilled in the art that the presentinvention, as described below, may be implemented in many differentembodiments. Any actual software code implementing the present inventionis not limiting of the present invention. Thus, the operational behaviorof the present invention will be described with the understanding thatmodifications and variations of the embodiments are possible, given thelevel of detail presented herein.

Advantages of Acoustic Impediography Sensing

As noted above, the present invention provides a more accurate andreliable alternative to the traditional biometric sensing systems. Moreparticularly, the present invention uses acoustic impediography as atechnique for sensing biometric, such as fingerprint ridges and valleys.

FIG. 1A is an illustration of the basic structure 100 of a fingerprint.In FIG. 1A, the basic fingerprint structure 100 includes ridges 102 andvalleys 104, which combine to form an entire fingerprint. FIG. 1B is anillustration of such a fingerprint 106. Thus, the present inventionutilizes principles of acoustic impediography to more accurately andreliably distinguish the ridges 102 from the valleys 104 associated withthe fingerprint 106.

The most significant advantage of acoustic impediography is a muchhigher contrast ratio, when compared with other modalities that rely,for example, on dielectric permittivity and thermal conductivity.

FIG. 2 is a tabular illustration 200 of comparisons of sensing tissueand air using dielectric permittivity, thermal conductivity, andacoustic impedance. Note that each technique is expressed in units ofmeasure uniquely associated with that particular technique.

In FIG. 2, for example, relative dielectric permittivity is expressed interms of a unit-less quantity 202. Thermal conductivity is expressed interms of watts (W) per meter (m)-centigrade (C) 204. Acoustic impedanceis expressed in terms of density (N) times the speed of sound per meter(m) 206. Each of these expressions is shown in terms of fingerprinttissue (i.e., ridge) column 208 and fingerprint air (i.e., valley)column 210. A contrast ratio column 212 compares the ridge column 208with the air column 210 for the respective dielectric, thermalconductivity, and acoustic impedance techniques.

As illustrated FIG. 2, acoustic impedance yields a significantimprovement in contrast ratio 212 in comparison to the other techniques.For example, the illustration 200 shows that the contrast ratio ofthermal conductivity in tissue compared to thermal conductivity of air,is about 8:1. The contrast ratio of dielectric permittivity in tissue tothe dielectric permittivity of air is about 32:1 (four times better thanthat of thermal conductivity). However, the contrast ratio of acousticimpedance in tissue to the acoustic impedance in air is about 4000:1,more than 100 times better than even the improved contrast ratio ofdielectric permittivity. Thus, as can be seen, acoustic impediography,having the higher contrast ratio, is inherently more capable ofdistinguishing ridges from valleys that thermal conductivity anddielectric permittivity.

FIG. 3 is a graphical illustration 300 that displays the superiorcontrast ratio of acoustic impediography, as discussed above in relationto FIG. 2, along a vertical axis 301. More particularly, a point 302represents the contrast ratio of thermal conductivity and a point 304represents the contrast ratio of dielectric permittivity, along the axis301. A point 306 represents the contrast ratio of acoustic impedancealong the vertical axis 301. As is graphically depicted in FIG. 3,acoustic impedance has a significantly better contrast ratio for tissueand air than dielectric permittivity and thermal conductivity. AlthoughFIGS. 2 and 3 only compare acoustic impedance with thermal conductivityand dielectric permittivity, acoustic impedance provides significantadvantages over other biometric sensing modalities.

The discussion above primarily focuses on the advantages of acousticimpediography as an inherently superior sensing technique to other knownsensing techniques. However, in order to apply the superiority ofimaging fingerprints using acoustic impediography in a practical manner,acoustic impediography based techniques must be incorporated intoconstructing sensing device and/or resonators.

Acoustic Impediography Based Sensing Devices

By way of background, attempts were made in the 1970s and 1980s to applyacoustic impediography sensing to imaging tissue samples in the medicalarena. These attempts, however, were only marginally successful, atbest, because in order to make this technique effective for medical use,ultra-sound signals had to be transmitted into tissue. Aftertransmission into tissue, one then had to observe the reflection andreconstruct from the reflection, an image of the tissue based uponvarious impedances throughout the tissue. In other words, these earlyapplications relied on acoustic impediography as a reconstructiontechnique as opposed to a sensing technique. These early applications,however, proved to be too error prone and ineffective forreconstructions and were largely abandoned.

Embodiments of the present invention, however, are able to effectivelyutilize acoustic impediography as a biometric sensing technique. Thisuse is possible in part because in the present invention, only tissuesurface features are of analyzed. The inventors of the subject matter ofthe present application have discovered that sensing these surfacefeatures (i.e., fingerprint ridges and valleys) using acousticimpediography, significant enhancements to biometric sensing andmeasurement can be derived.

More particularly, the present invention incorporates principles ofacoustic impediography into construction and use of mechanicalresonators/oscillators. For example, one embodiment of the presentinvention uses an arrangement of piezo pillars embedded in ainterstitional material suitable to fix them in place to form a matrixof piezo-electric resonators used to sense the ridges and valleys of thefingerprint.

By way of background, a pillar sensor matrix can be utilized, as anexample, for imaging fingerprint ridges and valleys. In exemplaryembodiments of the present invention, a sensor matrix comprises piezopillars embedded in a matrix material, such as e.g. polymer, forming a1-3 piezo composite structure. Using a crosshatched electrode pattern,pillars are excited by an electrical input signal, generating acousticwaves inside the pillar due to the inverse piezo-electrical effect. Ifthe sensor is constructed accordingly, the pillars' oscillations aremore or less damped by the ridge or valley of the fingerprint structurein physical contact with the sensors upper surface. The damping ishigher, for example, when exposed to a ridge and is lower if exposed toa valley. The corresponding physical property related to the damping isthe acoustic impedance, which is approximately 400 Rayl for the valley,and approx. 1,500,000 Rayl for the ridge.

The actual damping can be measured by monitoring the pillars mechanicaloscillations which are transferred into electrical current oscillationsvia the direct piezo-electrical effect.

It is equivalent to measure the mechanical Q (ratio of reactance toresistance in the equivalent electric series circuit representing themechanical vibrating resonant system) of the resonating pillar whichswitches from a higher value to a lower value if the acoustic loadswitches from a valley to a ridge. It is also equivalent to measure thecurrent, Ip, flowing through the element, if the acoustic load switchesfrom low to high. Higher loads are associated with lower Ip currents andlow loads with higher Ip currents. Using the impediography method asdescribed here, the impedance load on top of each pillar can beestimated in multiple ways from the pillar resonator property Qm. Forexample, the impedance load can be determined (i) analytically and (ii)by calibration.

In case i) an approximate equation for the minimum impedance of a singlepillar can be derived relating the pillar complex impedance to the topand bottom load conditions including the pillars electrical mechanicaland piezoelectric properties. in case ii) the pillars resonance propertye.g. the mechanical quality factor Qmn is estimated for various toploads Zn leading to a calibration curve from which for a given Qm thecorresponding unknown acoustical impedance of the load can be determinedusing a look up table (not shown). This look up table can be integratedinto the data processing flow, thus a quantitative map for the acousticimpedance is obtained from the individual pillar response across thepillar matrix.

However, for the fingerprint application it is not required to estimatethe surface acoustic impedance directly and quantitatively, it issufficient to observe a certain difference in the directly measuredproperty (Ip, Qm) to differentiate between ridge and valley.

It is important to avoid or minimize any lateral losses of acousticenergy (i.e., dissipating energy) from the longitudinal mode of pillaroscillation, preferably the rod extension mode, into lateral modes,generated from longitudinal modes due to lateral material coupling.These lateral modes will leak energy into the adjacent medium, intowhich the pillar is embedded. As the pillar oscillates in a longitudinalmode, shear waves are generated by the pillar side areas facing thematrix material. These shear waves are traveling away from the pillarcreating substantial loss of energy and hence these waves are dampingthe pillars oscillation. This process is discussed more fully below.

The spatial resolution of any fingerprint image obtained byimpediography is basically defined by the matrix structure, moreprecisely by the pillar pitch.

As a point of clarity, pillar matrix arrays can be manufactured using anumber of different approaches, such as:

1. Dice & fill

2. Injection molding

3. Soft molding

4. “Cookie cutter”

5. Micro Machined Ultrasound Transducer Technology (MUT-Technology)

6. Sol Gel process

7. Thick PZT-film laser lift off

Shear Wave Reflection

Aspects of the present invention, however, focus on matrix construction.One element of importance in this construction is considering the shearwave generation and propagation within the matrix material. Asunderstood by those of skill in the art, a shear wave is a type ofseismic wave that involves oscillation of a material perpendicular tothe direction in which the wave is traveling

Using experimental results and numerical finite element modeling, it canbe demonstrated that in case of matrix materials typically used for 1-3piezo composites, such as various types of polymers [referring to theCUE data base¹] the energy, leakage from the pillar of interest (poi)into the adjacent medium is so high that the pillar oscillation isdamped at an extent where the effect of the top load on pillar dampingbecomes small compared to the damping through the matrix. Thispropagation makes it more difficult to distinguish between ridges andvalleys. ¹CUE Materials Database R L O'Leary, G Hayward, G Smillie and AC S Parr The Centre for Ultrasonic Engineering, University ofStrathclyde, Glasgow, Scotland Version 1.2 Updated August 2005.

In the present application, techniques are provided to lower the effectof this shear wave leakage. In one such technique the pillar matrix isarranged in a manner such that the neighbor pillar reflects shear wavesback to the pillar from which they emerged.

This shear wave reflecting effect contributes to the pillar Q if thereflecting neighboring pillars are within a distance (curve) of awavelength of the shear wave generated by the poi. More specifically,the reflecting effect, and hence the pillars Q, is optimized if thedistance is a quarter wavelength of the shear wave. This effect isenhanced if the pillars are of square cross-section. In this way eachpillar of interest has four neighbors providing a parallel surfaceenhancing reflection.

FIG. 4A is an exemplary illustration of a 3×3 pillar matrix 400including pillars having a square cross-section. By way of example, thepillar matrix 400 includes a poi 402, along with adjacent neighboringpillars 404, 406, 408, and 410. As the poi 402 vibrates, shear waves 412are generated and reflected into the adjacent neighboring pillars 404,406, 408, and 410, and also into a corner pillar 414. Conversely, shearwaves 416 and 418 are generated and are reflected off of the cornerpillar 414 and reflected back into the poi 402. This corner reflectionaspect is discussed more fully below.

If any of the neighboring pillars 404, 406, 408, or 410 oscillates atthe same time as the poi 402, the shear waves from these neighborsdirectly reflecting on the poi 402 potentially interfere negatively withits ongoing oscillation. The inventor of the present application hasdiscovered, however, that this negative effect can be substantiallyreduced. The present application demonstrates that if the pillars areelectrically connected diagonally, with respect to the matrixorientation as illustrated in FIG. 4B, as opposed to vertically orhorizontally, this negative effect can be reduced.

For example, FIG. 4B is an illustration of an exemplary matrix 420having a poi 422 connected diagonally with surrounding pillars 424 and426. In FIG. 4B, although the poi 422 is surrounded by four pillars 428,430, 432, and 434 face to face, the poi 422 is not electricallyconnected to any of these four pillars. Instead, the poi 422 iselectrically connected to the pillars 424 and 426. Since the pillars428, 430, 432, and 434 are not connected to the poi 422, these pillarsare inactive, and hence provide only passive reflection in relation tothe poi 422. These pillars no longer directly reflect into the poi 422.

If pillars along a diagonal line are driven, such as the pillars 402,414, and 415 in FIG. 4A, the active pillars of the matrix are cornerelements (414 and 415). In FIG. 4A, shear waves 416 and 418, which arebended, are emerging from these corner elements 414 and 415 and areimpinging on the corners of the poi 402. However these shear waves 416and 418 do not have constant phase because they are bended. Becausethese waves lack constant phase, because they must travel a greaterdistance to reach the poi 402, and the fact that a portion of theirenergy is dissipated along the side of the pillars 404 and 406, theireffect on the Q of the poi 402 is reduced.

FIG. 4C is an illustration of the matrix of FIG. 4B having passivesurrounding pillars. That is, FIG. 4C is an illustration of directtransmission of shear waves overlapping with passive reflections fromneighboring elements.

Damping, through energy leakage from the poi into the surrounding matrixmaterial, is reduced if the acoustic impedance of the material becomeslower. If the acoustic impedance would be as low as the air's acousticimpedance the leakage can be completely neglected and this would be theideal condition for achieving a high resonator q of the mechanicaloscillation. But then the pillars cannot be kept in place. Therefore, aminimum of adhesive force between pillar and matrix material is requiredproviding sufficient stability for operation and during processing the1-3 piezo substrate for sensor applications. A suitable candidate for amatrix material exhibiting substantially lower acoustic impedance couldbe porous polymers.

That is, for example nm sized gas bubbles immersed into polymer duringthe fluid polymer state. After polymer-bubble mixture is poured aroundthe pillar matrix of the 1-3 composite it will harden to a very lowdensity interstitional material.

As noted above, a piezo-electric resonator represents one type ofmechanical resonator. All mechanical resonators, regardless of the type,are excited or driven via application of some type of load or force.There are many different ways that such a force can be applied.

By way of background, a resonator is defined by a quality factor Qm, orits quality of resonance. If the resonance is excellent (i.e., very lowdamping), the resonator can be described as a very high Q resonator. Ifthe resonator is not quite as good, it might be low Q resonator. Forexample, a typical oscillator (e.g., a quad oscillator) has a Q ofroughly 200,000 to 1 million. The Q of resonators used in embodiments ofthe present invention is only about 30, to 40, etc. When the resonatorsare touched, which is the basic idea of acoustic impedance sensing,acoustic impedance provides friction to the oscillator. However, becauseof that friction, the resonator is damped. When the resonator is damped,it looses energy. The energy loss is expressed as a change of Q. Thechange of Q, in turn, is used to detect changes in the impedance. Forexample, the mechanical Q of a resonating pillar switches from a highervalue to a lower value when the acoustic load switches from a valley toa ridge.

FIG. 5 is an illustration of acoustic impedance sensing system 500constructed in accordance with an embodiment of the present invention.In the present invention, sensing of an acoustic impedance object 501(e.g., a finger) is based upon three essential variables: an excitingforce 502, resonator scanning principles 504, and measured properties506 that determine the degree of damping, the damping observationmechanism, and the manner of damping.

In FIG. 5, a mechanical resonator/oscillator 508 can operate inaccordance with mechanical or electrical scanning principles 504, or acombination of both. Application of an initial oscillation push canoccur via one of the excitation forces 502. These initial excitationforces 502 include, as a few examples, piezo-electric, electrical,mechanical, optical, thermal, radio frequency (RF) based, and magnetic.The mechanical resonator 508 can be driven by a number of differentforces. Once the mechanical resonator 508 begins to oscillate, itsvibration can be measured based upon a number of different properties506.

The measurement properties 506 can include, by way of example, voltage,current, impedance, displacement, stress, strain, optical reflection,along with many others.

A matching layer 510 is provided to reduce reflection of energy passedfrom the acoustic impedance object 501 into the mechanical resonator508. In the absence of the matching layer 510, reflection can be as highas 90%. The matching layer 510 can be of any suitable thickness, matchedto predetermined acoustic impedance values.

FIG. 6 is an exemplary method 600 of practicing an embodiment of thepresent invention. In the method 600, a fingerprint is captured andstored in a computer memory in step 602. In step 604, the storedfingerprint is analyzed using acoustic sensing principles.

Conclusion

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be understood by those skilledin the relevant art(s) that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined in the appended claims. Accordingly, the breadthand scope of the present invention should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with the following claims and their equivalents.

1. A sensor, comprising: one or more mechanical devices for capturing animage of a fingerprint, each device including a matrix of pillars;wherein a pillar of interest within the matrix is (a) only connected todiagonally adjacent pillars and (b) devoid of a connection to verticallyand horizontally adjacent pillars.
 2. The sensor of claim 1, wherein thepillars are formed of at least one of resonators and oscillators.
 3. Thesensor of claim 1, wherein a q factor of individual pillars within thematrix is optimized by adjusting a distance between pillars inaccordance with a quarter shear wavelength at an operating wavelength.4. The sensor of claim 3, wherein the q factor can be improved byreducing an acoustic impedance of matrix material, the reducing beingfacilitated by at least one of (i) reducing a density of the matrixmaterial and (ii) reducing a stiffness of the matrix material.
 5. Thesensor of claim 4, wherein the sensor is configured to be excited viapiezo-electric, magnetic, or electrical forces.
 6. The sensor of claim5, wherein the fingerprint image is captured via scanning a fingerprintstructure in a manner that includes at least one from the groupincluding mechanically and electronically.
 7. The sensor of claim 6,wherein damping is observed using at least one from the group includingvoltage, current, impedance, displacement, stress, strain, and opticalreflection.
 8. The sensor of claim 7, further comprising a matchinglayer configured for reducing a difference in acoustic impedance betweenthe sensor and a structure of the finger.
 9. The sensor of claim 7,wherein the matching layer includes a predetermined thickness andacoustic impedance.
 10. The sensor of claim 1, wherein the pillar ofinterest is electrically connected to the diagonally adjacent pillars.11. The sensor of claim 1, wherein the vertically and horizontallyadjacent pillars are inactive in relation to the pillar of interest. 12.The sensor of claim 1, wherein the vertically and horizontally adjacentpillars provide only passive reflection in relation to the pillar ofinterest.
 13. The sensor of claim 1, wherein the pillars are of a squarecross-section.
 14. The sensor of claim 13, wherein the pillar ofinterest is surrounded by four pillars face to face and is devoid of aconnection to the four pillars.
 15. A method comprising: capturing animage of a fingerprint using one or more mechanical devices, each deviceincluding a matrix of pillars; wherein a pillar of interest within thematrix is (a) only connected to diagonally adjacent pillars and (b)devoid of a connection to vertically and horizontally adjacent pillars;and monitoring excitation of pillars adjacent to a pillar of interest,the adjacent pillars being (a) only diagonally connected to the pillarof interest and (b) devoid of a vertical or horizontal connection to thepillar of interest.